Calendar year is a bad idea. I would suggest August 1st or September 1st cutoff. That way everyone can play on grade who is of age. It is a big advantage holding back. The key to soccer is the player cards with birthdates. Lacrosse and ice hockey in the USA with the paucity of excellent training makes the holdbacks impact even more extreme compared to football, hoops and baseball. 20% of the top teams with the best training are holdbacks? Sooner you holdback the better for the individual you go from good on your on age team to star/ featured player on your team. You get 6 years of being a featured player/ most attention and training. The "super/ premier" clubs would resist this movement to go to all playing same age cohort as I think they feature the most holdbacks and the current model is working for them.